The invention lies in the field of processes for the recovery of aluminum from its ores, and/or the manufacture of potassium hydroxide from potassium sulfate.
The present invention broadly comprises the manufacture of potassium hydroxide from potassium sulfate by electrodialysis. An improvement of the broad process is the use of sodium hydroxide in the electrodialysis process. The invention is peculiarly adapted for use with the basic process disclosed in the two above-mentioned patents, as the improvement utilizes byproduct potassium sulfate and sodium hydroxide from the bleed stream from the Bayer leach of the basic process to manufacture potassium hydroxide by electrodialysis for use in leaching the roasted alunite ore calcine.
The basic process is defined by the following process steps, some of which are optional, as illustrated in the specification and flow diagram of the referenced patents.
(1) ROASTING PARTICULATE ALUNITE CONCENTRATE TO REMOVE WATER OF HYDRATION;
(2) REMOVING SULFUR COMPOUNDS AND ALKALI METAL COMPOUNDS, INCLUDING POTASSIUM SULFATE, FROM THE ROASTED ORE EITHER BY THE PROCEDURE OF U.S. Pat. No. 3,890,425 in which the roasted ore is subjected to a reducing roast and then to an oxidizing roast followed by leaching with potassium hydroxide, or by the process of U.S. Pat. No. 3,890,426 in which the roasted ore is leached directly with alkaline hydroxide, each alternate procedure terminating in a leaching step and leaving a residue containing aluminum values and a slurry of solution containing dissolved potassium sulfate;
(3) RECOVERING POTASSIUM SULFATE FROM THE LEACH SOLUTION OF (2) BY CRYSTALLIZATION, AND RECYCLING THE MOTHER LIQUOR TO THE LEACHING STEP WHILE BLEEDING A STREAM THEREFROM;
(4) DIGESTING THE RESIDUE FROM THE LEACHING STEP WITH ALKALI METAL HYDROXIDE SOLUTION TO CONVERT ALUMINUM VALUES IN THE RESIDUE TO SOLUBLE ALUMINATE;
(5) PRECIPITATING SILICA FROM THE RESULTING SOLUTION OF STEP (4) TO LEAVE A SOLUTION OF AUMINATE AND A RESIDUE KNOWN AS A DESILICATION PRODUCT CONSISTING ESSENTIALLY OF SODIUM ALUMINUM SILICATE ALONG WITH OTHER IMPURITIES;
(6) PRECIPITATING ALUMINUM VALUES FROM THE SOLUTION OF STEP (5);
(7) RECYCLING THE MOTHER LIQUOR FROM STEP (6) TO THE DIGESTING STEP WHILE REMOVING A BLEED STREAM THEREFROM, AND
(8) RECOVERING PRODUCT ALUMINUM FROM THE ALUMINUM VALUES OF STEP (6).
The expression "roasting and leaching to remove water and compounds of sulfur and alkali metals" as used herein includes the two procedures of step (2) above.
Processes from the production of aluminum from alunite must be as economical as possible in order to produce a commercial grade product which is competitive with the product produced from bauxite. This necessarily means utilization of byproducts wherever possible. Potassium hydroxide is the preferable leaching agent for leaching the calcined alunite ore of the basic process, and this leaching agent must ordinarily be purchased commercially. Conversion of byproducts from the basic process into this essential and somewhat expensive chemical for the above leaching step is a desired objective.
Accordingly, it is an object of this invention to provide a process for the economic utilization of the byproduct potassium sulfate and bleed stream sodium hydroxide from the basic process. A further object of the invention is to provide a process for the production of potassium hydroxide from potassium sulfate by electrodialysis with or without the use of sodium hydroxide.